1. Field of the Invention
The present invention relates to an electrographic image forming apparatus (hereinafter, simply called an “image forming apparatus), such as a copying machine, a printer, facsimile terminal equipment, and a multifunction printer, which forms images using an electrophotographic system. Especially, the present invention relates to an electrophotographic image forming apparatus using dark toners and light toners, which have a same hue, but are different from each other in density.
2. Description of the Related Art
The image forming apparatus forming color images forms four color toner images on, for example, a photosensitive drum of an image bearing member, wherein the four colors include Y (yellow), M (magenta), C (cyan), and K (black). The above-mentioned toner images are sequentially superimposed and transferred on, for example, a sheet (transfer material) held on a transfer drum (transfer film). In the above-mentioned case, an electrostatic latent image of, for example, cyan as a first color is formed on a photosensitive drum, based on input signals including read image information, and then a C toner image obtained by developing the cyan latent image is transferred on a sheet on the transfer drum. The above-mentioned series of transfer processes are sequentially repeated for other three colors of Y toners, M toners, and K toners as a second color, a third color, and a fourth color in this order, thereby a color image is obtained.
Recently, latent images are collected and formed on the drum surface of the photosensitive drum, wherein the drum surface bears dots of a predetermined potential, and a solid portion, a half-tone portion, and a line portion are expressed by changing the density of the dots in the image forming apparatus using digital image signals. However, in the above-mentioned case, toner particles are hardly placed on the dots, and there is caused a state in which the toner particles deviate from the dots. Thereby, it is difficult to obtain the gradations of a toner image corresponding to the dot density ratio between the black portions and the white portions of a digital latent image. Moreover, when resolution is improved by smaller dots in order to improve image quality, it becomes more difficult to reproduce a latent image formed by a collection of microdots. Especially, the resolution and the gradations in highlight portions are deteriorated to cause a tendency to lose sharpness in the color of the image. Moreover, disturbance of the dots causes a sense of granularity which leads to reduction in the image quality for the highlight portions, and image unevenness by the granularity is an unpredictable unstable element of the image quality.
On the other hand, an inkjet recording method is a simple system as can be seen in a technology processing dark-colored ink and light-colored ink, which has been disclosed in, for example, Japanese Patent Application Laid-Open No. 58-039468. Moreover, the inkjet recording method has been supported by use of recent and excellent high-quality specialized-paper and does not cause the above-described problems raised by the electrophotographic system. Furthermore, the inkjet recording method has a unique advantage that there is caused no sense of granularity, because the method used dark-colored ink and light-colored ink. As excellent performances are obtained by use of, especially, light-colored ink, an electrophotographic system applying light-colored ink would result in a largely improved system.
Furthermore, even with regard to optical dot gain which is a barrier for developing an electrophotographic system forming a high-quality image, an idea of introducing light-colored toners is effective for solving the problems caused by use of micro toners. Based on the above idea, there has been proposed an image forming apparatus (refer to, for example, Japanese Patent Application Laid-Open Nos. 11-84764 and 2000-305339) forming an image by combining a plurality of toners which are different from one another in density, for example, by using light-colored toners (light toners) in the highlight portions, and dark-colored toners (dark toners) in solid portions. Moreover, there has been proposed an image forming apparatus (refer to, for example, Japanese Patent Application Laid-Open No. 2000-347476) in which dark toners and light toners having a maximum reflection density below half of the maximum reflection density of each of the dark toners are combined. Moreover, there has been proposed an image forming apparatus (refer to, for example, Japanese Patent Application Laid-Open No. 2000-231276) having a configuration in which dark toners with an image density of 1.0 or more and light toners with an image density less than 1.0 are combined, when toner quantity on a sheet is 0.5 mg/cm2. Furthermore, there has been proposed an image forming apparatus (refer to, for example, Japanese Patent Application Laid-Open No. 2001-290319) in which toners are combined so that a recording density ratio between the dark toner and the light toner is adjusted at 0.2 through 0.5.
In the image forming apparatuses disclosed in the above-described patent documents, it has been assumed that plain paper is mainly used as a kind for a sheet as the transfer material. As described above, the inkjet recording method has realized a high-level image forming technology by using high-quality specialized paper with excellent printing performances based on stable high-resolution binary recording. It is very difficult to introduce the image forming technology as it is based on the above-described inkjet recording method into the electrophotographic system which has mainly used plain paper. Conventionally, the electrophotographic system has improved density gradations by using a low resolution screen which has been used for printing, in order to reduce a performance difference between high quality specialized paper and plain paper. Thereby, problems, such as coarseness and granularity, peculiar to the electrophotographic system may be solved by using light toners in low density portions.
Incidentally, greater importance is often placed on smoothness in highlight portions as an image using dark toners and light toners when the quality of an image like a picture is improved.
However, toner quantity placed in highlight portions is increased as a result of smoothing the highlight portions. Moreover, when a toner image is formed on a sheet (recording material) with a rough surface, transfer unevenness of light toners in highlight portions is easily caused in a toner image with an increased quantity of toner.